One conventional screw type linear actuator, is shown in FIG. 3. The illustrated actuator is provided with a hollow inner cylinder 22 which is guided slidably within an outer cylinder 21. A nut 23, which is fixed on the base end side of the inner cylinder 22, is in threaded engagement with a screw shaft 24 which is disposed coaxially within the inner cylinder 22. In the peripheral portion of the nut 23 is formed a guide hole 23A for passing therethrough of a tie rod 25, the tie rod 25 being mounted axially within the outer cylinder 21.
When the screw shaft 24 is driven rotatively by means of a motor 26 through a reduction mechanism 27, the nut 23 engaged threadedly with the screw shaft 24 moves axially of the screw shaft because its rotation is prevented by the tie rod 25, so that the inner cylinder 22 moves forward or backward relative to the outer cylinder 21.
Another structure is disclosed in Japanese Utility Model Publication No. 50600/95. In this structure, an outer cylinder is formed using a square pipe, and tie rods are disposed at the four corners in the interior of the square pipe. Further, the outer surface of a nut engaged threadedly with a screw shaft is formed in a square shape in conformity with the inner surface of the outer cylinder. In the four corners of the nut are formed cutout portions for engagement with the tie rod.
In the screw-type linear actuator of the structure shown in FIG. 3, however, the tie rod 25 is apt to deflect or oscillate due to a rotational torque acting on the nut 23 from the screw shaft 24, so that when the stroke of the inner cylinder 22 is long, it becomes difficult for the inner cylinder 22 to perform a smooth advancing and retreating motion.
According to the structure disclosed in the foregoing Japanese Utility Model Publication 50600/95, the torque acting on the nut with rotation of the screw shaft is borne by not only the tie rod but also the inner wall surfaces of the outer cylinder to diminish deflection of the tie rod. However, since the outer cylinder has a square section, it is necessary that the inner cylinder bearing portion on the front end side of the outer cylinder and the socket portion on the body side which fixes the base end portion of the outer cylinder be formed in a square sectional shape conforming to that of the outer cylinder. For enhancing both accuracy and sealability, such a requirement gives rise to a problem in machining and also to a structural difficulty. An increase of production cost also results.